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It's a question I've been struggling with lately. Downvotes reinforce groupthink, that much is subjectively clear. I've considered solutions, and I believe forcing people to reply before they are allowed to downvote would help. However, in opposition to my own idea, I consider that simply requiring a more effort wouldn't necessarily change the groupthink problem, but would just stifle low effort downvotes. Let's cons…
We could remove downvotes. They had three purposes: 1) Crowdsourced moderation. Well, everyone is using manual moderation anyway. 2) Silencing others. This is a misfeature. 3) Finding the best content. Upvotes are enough for that. That said, I'm not sure this will reduce groupthink. Twitter has no downvotes and plenty of groupthink.
First downvote can be used for arguments like "let's increase taxes to the rich because, this way we can build more fair society, European countries have shown that high taxes..."
Second downvote can be used for comments like "Let's tax these rich bastards".
I want to also mention reddit voting system which I kinda like: they have downvoting, but they also have a sorting by "controvervial": top comments are the comments with high number of both upvotes and downvotes, such comments really help to see the full spectre of opinions in discussions, while the universally true and not interesting comments everyone agrees with like "we should not kill people", are below controversial comments.